Implement filter for secondary particle production binned by energy (#3453)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

include/openmc/constants.h

@@ -301,7 +301,7 @@ enum class TallyEstimator { ANALOG, TRACKLENGTH, COLLISION };
 enum class TallyEvent { SURFACE, LATTICE, KILL, SCATTER, ABSORB };
 
 // Tally score type -- if you change these, make sure you also update the
-// _SCORES dictionary in openmc/capi/tally.py
+// _SCORES dictionary in openmc/lib/tally.py
 //
 // These are kept as a normal enum and made negative, since variables which
 // store one of these enum values usually also may be responsible for storing

include/openmc/particle_data.h

@@ -535,6 +535,11 @@ private:
 
   vector<SourceSite> secondary_bank_;
 
+  // Keep track of how many secondary particles were created in the collision
+  // and what the starting index is in the secondary bank for this particle
+  int n_secondaries_ {0};
+  int secondary_bank_index_ {0};
+
   int64_t current_work_;
 
   vector<double> flux_derivs_;
@@ -689,7 +694,20 @@ public:
 
   // secondary particle bank
   SourceSite& secondary_bank(int i) { return secondary_bank_[i]; }
+  const SourceSite& secondary_bank(int i) const { return secondary_bank_[i]; }
   decltype(secondary_bank_)& secondary_bank() { return secondary_bank_; }
+  decltype(secondary_bank_) const& secondary_bank() const
+  {
+    return secondary_bank_;
+  }
+
+  // Number of secondaries created in a collision
+  int& n_secondaries() { return n_secondaries_; }
+  const int& n_secondaries() const { return n_secondaries_; }
+
+  // Starting index in secondary bank for this collision
+  int& secondary_bank_index() { return secondary_bank_index_; }
+  const int& secondary_bank_index() const { return secondary_bank_index_; }
 
   // Current simulation work index
   int64_t& current_work() { return current_work_; }

include/openmc/tallies/filter.h

@@ -39,6 +39,7 @@ enum class FilterType {
   MUSURFACE,
   PARENT_NUCLIDE,
   PARTICLE,
+  PARTICLE_PRODUCTION,
   POLAR,
   SPHERICAL_HARMONICS,
   SPATIAL_LEGENDRE,

include/openmc/tallies/filter_energy.h

@@ -1,6 +1,7 @@
 #ifndef OPENMC_TALLIES_FILTER_ENERGY_H
 #define OPENMC_TALLIES_FILTER_ENERGY_H
 
+#include "openmc/particle.h"
 #include "openmc/span.h"
 #include "openmc/tallies/filter.h"
 #include "openmc/vector.h"
@@ -72,5 +73,36 @@ public:
   std::string text_label(int bin) const override;
 };
 
+//==============================================================================
+//! Bins the outgoing energy of secondary particles
+//!
+//! This filter can be used to get the photon production matrix for multigroup
+//! photon transport, the energy distribution of secondary neutrons, etc. Unlike
+//! other energy filters, the weight that is applied is equal to the weight of
+//! the secondary particle. Thus, to get secondary production it should be used
+//! in conjunction with the "events" score.
+//==============================================================================
+
+class ParticleProductionFilter : public EnergyFilter {
+public:
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "particleproduction"; }
+  FilterType type() const override { return FilterType::PARTICLE_PRODUCTION; }
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  std::string text_label(int bin) const override;
+
+  void from_xml(pugi::xml_node node) override;
+
+  void to_statepoint(hid_t filter_group) const override;
+
+protected:
+  ParticleType secondary_type_; //!< Type of secondary particle to filter
+};
+
 } // namespace openmc
 #endif // OPENMC_TALLIES_FILTER_ENERGY_H